The present invention relates to string digital-to-analog converters (string DACs) and associated interpolation circuits. Other aspects of the invention relate to circuitry and to techniques for calibrating interpolation string DACs to correct integral nonlinearity (INL) errors.
Monotonicity and low INL are usually required in applications in which DACs, including string DACs, are used. The INL error is the difference between the actual output of a. DAC and an “ideal” output of the DAC.
A string DAC is a type of static DAC that includes a plurality of resistors connected in series between a high reference voltage and a low reference voltage, wherein the various connecting nodes between the resistors constitute tap points that are selectively switched to an output node in response to a digital input. The voltage of the tap point is selectively switched to an output node, and is an inherently monotonic analog representation of the digital input. For an N-bit DAC, 2N−1 tap point voltage levels are required, so 2N resistors are required for a string DAC. This number of resistors is too large to be practical for more than approximately eight to ten bits, unless the DAC is segmented into a string DAC section and an interpolation DAC section, in which case the interpolation section also must be inherently monotonic to ensure that the entire DAC is monotonic.